Some restorers may feel that the modifications
carried out by Ranulph to improve the sound quality of the this set are a little
excessive. However the modifications could be reversed at a later date if required,
and Ranulph clearly wants a set that is pleasant to listen to with today's broadcasts.
The subject of modifications is discussed on the Modifications
page in the Repair and Restoration Information section of this website.

General Description

This
set, released in August 1955, is a first generation AM/FM table receiver. It
was bought for £5 at the local junk shop. Although dirty and slightly scratched,
it seemed in basically sound condition; there was little evidence of previous
‘repair’ work, except that the double-triode of the FM front-end was missing.
Fortunately, the wiring was all PVC insulated and would not need replacement.

The case is of simple construction: the sides,
top and bottom are made of rectangular veneered 3/8" plywood; the front
is a cloth-covered loudspeaker baffle with a full-width glass tuning scale beneath.
Some beading covers the join between the sides and the baffle/tuning scale.
The back and inspection cover underneath are of perforated cardboard. A 6 ½"
loudspeaker is fitted to the baffle.

The electronic design is very standard for sets
of this era; it includes a permeability tuned ECC85 FM front-end and magic eye
tuning indicator. The audio amplifier has anode-to-anode feedback, designed
to provide additional bass boost on FM. The wavechange switch and ECH81 circuitry
are built on to a subassembly, alas making access to the associated components
difficult. Generally, the standard of construction is cost-effective but of
reasonable quality.

10 nF polyester capacitors placed across HT and heater
supplies; 10 nF polypropylene capacitor connected between live and neutral,
and between neutral and earth, on transformer side of mains switch (RF interference
can be a problem in my area).

Wavechange switch moved back by 1/16" to ensure correct
alignment of tuning drive pulley (to prevent cord knotting itself on the pulley).

10 M resistor connected between pin 8 of V3 and junction
of R21, C47 and R26, to improve linearity of AM detector.

2.2 pF capacitor connected between pin 7 of V3 and
pin 3 of V4, to improve linearity of FM discriminator.

R25 reduced to 180K, to improve linearity of audio amplifier.

500 pF capacitor added between ground and junction
of R20 and C50, to provide FM de-emphasis.

Performance

The AM and FM distortion was checked by applying
a modulated signal to the aerial input and measuring the output on the loudspeaker
terminals at a level of 1V RMS. The FM distortion was about 0.7% at 100 kHz
peak deviation, and the AM distortion 1% at 95% modulation depth.

2W of audio power were available at the onset
of clipping.

The AM bandwidth was about +/- 9 kHz at
-3 dB. (This is rather wide for today’s circumstances: broadcasters restrict
the modulation bandwidth to 5 kHz.)

The FM sensitivity was about 100 mV to achieve
limiting. The audio output remained usable well below this level.

Explanatory Notes

Unfortunately, the output transformer didn't survive initial
testing. When I took it apart, there were about 3 breaks and some evidence of
corrosion. I had a lot of fun winding 3,500 turns of 40 swg wire in neat layers!
Fortunately, the insulation of modern wire is much better, and you don't need
to worry too much if adjacent layers touch. I used PTFE plumber's tape between
the layers - it seems to have ideal electrical and physical characteristics.
In another guise, I repair antique clocks: a mainspring winder - really just
a handle and spindle mounted in a frame - is useful for coil-winding. It would
be easy for the amateur wireless restorer to make a similar arrangement.

I didn't measure the distortion introduced by the FM discriminator,
but it must have been greater than 10% at full deviation. Increasing the coupling
as mentioned in the summary reduced it to well below 1%, and had the added advantage
of making the tuning of the set much less critical.

The manufacturers had forgotten to add de-emphasise, and the
sound was bright, to say the least. In an attempt to compensate, they had added
a rather nasty bass boost circuit, which started to work at about 1 kHz. With
the correct de-emphasise, the bass boost can be removed, or arranged to work
at a much lower frequency where its effect is more pleasant. The tone control
was another horror. It only had an effect - an excessive one - during the first
few degrees of rotation. With an external low-impedance input (through the Gram
sockets) it had no effect at all. (A quick look at the circuit diagram shows
why.) It can be made usable very simply as detailed in the repair summary.

When aligning the FM front-end, it is not a good idea to adjust
the beehive capacitors with your fingers: they have the full HT on them! The
first IF transformer (part of the front-end) is best aligned by loosely coupling
the IF signal into the grid of the FM mixer. The alignment will need to be checked
again if the neutralising capacitor C11 is adjusted.

The AM detector introduced severe distortion above 65% mod
depth. This was the result of the additional AC loading imposed by the volume
control and the AGC. (With AM-only sets, the AGC was usually derived from a
second diode, and AC loading was less of a problem.) A small amount of forward
bias (via a 10 M resistor from the HT supply) helps greatly. Ideally, the bias
should be proportional to signal level, and it is possible to arrange this by
returning the 10 M resistor to the screen grid of the IF stage. At low signal
levels, the screen voltage is also low. As the AGC takes effect, and the current
through the valve falls, the voltage rises towards the supply rail. The AM IF
bandwidth of +/-9 kHz is rather too wide by today's standards, as the broadcasts
are restricted to +/-5 kHz. However, I didn't attempt to alter this.

The service sheet recommends aligning the AM front-end by
applying a signal to the aerial input through a 200 pF capacitor. This value
is much too large, and heavily loads the RF tuned circuits. It is much better
to plug in the aerial that the set would normally use, and loosely couple the
output of the signal generator into this.

By far the most troublesome problem was leakage across the
wavechange switch. Somehow it had become coated with a conductive film, which
produced a variety of unpleasant and obscure symptoms. Cleaning the surfaces
of the switch wafers adequately was difficult (to say the least) because of
the limited accessibility.

Anyway, the set now works pretty well. If nothing else, the
effort was educational!

Update

I received the following information about the repair of another
of these sets from Richard Newman

I read with great interest one of your 'recent repairs'
regarding a Baird 301. I have had one of these for some time and have just
got round to working on it.

Like the report, I too changed all the Hunts capacitors
and fitted correct values where incorrect substitutes had been made. Before
all this the set worked but sounded rather 'wooly'. I found that a lot of
the Hunts caps had changed in value. One in particular in the de-emphasise
had gone from .01uF up to around 0.4uF so that would account for the wooly
sound on FM. Also for some strange reason, the tone correction cap had been
changed from .002uF to 18000pF.

Once these had been changed, the FM sound was superb.
I don't consider it to be too 'bright' as in the report. I haven't carried
out any of the modifications suggested by your contributor as I don't consider
them necessary.... plus it detracts from the original design.

The only thing I would agree with is the tone control
which is just as described. However I have left it as is because it doesn't
bother me. Perhaps the speaker I am using is less 'toppy' than the one used
in the report. Whatever the reason, I am very happy with the sound of my own
Baird 301 I just thought you might be interested in my own findings.